The reactors or adsorbers that are now used are becoming increasingly bulky to handle an increasingly large demand for the desired product.
Furthermore, the desired product should reach a purity that exceeds 99.5%, which is not a priori compatible with the volume of the batch that is to be processed and therefore with very large reactor capacities.
The technological background that illustrates the use of an adsorption device with simulated countercurrent is described in U.S. Pat. No. 2,985,589.
This device comprises at least one cylindrical column that contains an overall cylindrical solid mass and has an approximately annular section.
A main fluid that is introduced by a pump flows through the solid bed along the central axis of the column, according to a flow that it is desired to designate a piston type flow (plug flow). In other words, the fluid should have a composition and a front flow that are uniform at all points of the section of the column.
A device such as that described in U.S. Pat. Nos. 3,214,247 and 4,378,292, which are included as references, makes it possible to accomplish this objective. It generally includes a number of beds of an adsorbent, supplied by a number of distributor plates, whereby each bed is supported by an upper grid that is approximately perpendicular to the axis of the reactor and makes it possible for the fluid to flow. Each plate is divided into sectors, and each segment of the distributor plate comprises two deflectors that are unperforated, flat, or overall tapered (of variable thickness) and that are arranged on the same horizontal plane, between which a circulation space for the fluid is arranged. A lower grid under the deflectors makes it possible to distribute the fluid uniformly in the lower bed of adsorbent.
At each distribution plate, at least four transfer lines for secondary fluids (batch injection line, desorbant injection line, line for drawing off an extract, and line for drawing off a refined product) that include a set of valves are connected to means for switching this set of valves.
The injection and draw-off of these fluids are accomplished between certain beds that define zones, and within a regular space of time called period T the points of introduction and draw-off that delimit the zones of the interval between beds (C.sub.k) and (c.sub.k+1) are moved by the interval between beds (c.sub.k+1) and (c.sub.k+2).
If n is the number of beds, n.times.T defines the cycle period.
A recycling pump recycles the fluid from the bottom of the column to the top.
The secondary fluids (batch or desorbant) are introduced or drawn off (extract, refined product) into or from the circulation space via an introduction or draw-off chamber that is pierced with openings.
Each distributor plate can be divided into sectors. According to U.S. Pat. No. 3,789,989, each plate sector, which is delimited by radial walls, comprises a chamber for introducing or drawing off secondary fluid.
In the case where the distributor plate of each sector contains only a single chamber, each chamber of a given sector is connected by a pipe to a single supply or draw-off line that is connected to the outside of the column.
According to Patent Application EP-A-769316, each secondary fluid is introduced or drawn off via its own introduction or draw-off chamber, which uses a number of openings opposite the circulation space. The upper and lower walls of these chambers constitute the deflectors that are mentioned above. Therefore, when the distributor plate of each sector includes several chambers, each chamber of a bed sector is connected by a pipe to a line that is intended to receive only a single fluid either to supply the appropriate chamber with desorbant or with batch or to draw off from the appropriate chamber the refined product or the extract. Thus, for example, if each sector comprises four chambers, one intended for the batch, the second for desorbant, the third for refined product, and the fourth for extract, the chamber CF of the given sector that receives batch F will be connected to a line that receives all of the pipes of different chambers CF pertaining to the same adsorbent bed.
In a paraxylene separation unit that operates with a simulated fluid bed and that comprises two adsorbers that are arranged in series of twelve beds each with a molecular sieve, a deformation (or drag) of the longitudinal concentration sections, which is reflected by a lack of performance relative to the ideal performance expected, has been noted.
In particular, the drag of the concentration in impurities at the draw-off of the extract is reflected by a significant reduction in the purity of the extract (less than 99%) relative to the purity expected (greater than 99.5%).
The analysis of the problem that was performed on the separation unit showed that these deformations (or drags) of the longitudinal concentration sections were due to parasitic circulations through each of the distribution chambers that are arranged in the sectors of each plate during periods when fluid is neither introduced nor drawn off through the chamber in question.
This is, in particular, wow, i.e., the exchange of material due to turbulence at the openings of the distribution chambers between the main fluid which circulates in the circulation space and the fluid that is contained in the chambers. This phenomenon is known for producing weak drag.
This is also mainly recirculation of a distribution chamber of a plate sector to the chamber that is similar to another sector of the same plate, via the coupling pipe that connects these chambers to one another and to the line for transfer to the outside of the adsorber.
This recirculation is due to small pressure differences that exist between the sectors of the same plate. In theory, this pressure should be the same throughout the same plate. In practice, small differences exist because of various imperfections such as the imperfections of flow of the main fluid through the adsorbent beds and, for periods when secondary fluid is neither introduced nor drawn off in a chamber, this induces a recirculation of a portion of the main fluid that is picked up at the circulation space of a sector where the pressure is higher, to the circulation space of a sector where the pressure is lower via the openings of the chambers in question.
The portion of main fluid that is recirculated enters one of the chambers by passing through the openings of the chamber in question that belongs to the highest pressure sector.
This portion of fluid then advances to the similar chamber that belongs to the lower pressure sector via the coupling pipe that connects these chambers to one another.
Finally, this portion of the fluid joins the main fluid in the circulation space of the lower pressure sector by passing through the openings in the chamber of this sector.
The recirculation flow between two sectors of the same plate is a function of the pressure differences that exist between these two sectors, as well as of the size of the openings of the chambers of the sectors in question.
The dwell time of the fluid that recirculates in this way from a chamber of a sector to the corresponding chamber of another sector is itself a function of the volume of the source and destination chambers, of the volume of the pipe that connects them, and of the recirculation flow between these chambers.
If the plate comprises multiple sectors, there will be a combined general recirculation from the sectors where the pressure is the highest to the sectors where the pressure is the lowest, with this recirculation being accomplished with a mean overall dwell time TR.
In this unit that operates in a simulated fluid bed, the composition of the main fluid at a plate varies constantly as a function of time. This is due to the advance of the longitudinal concentration section, which moves under the action of the circulation of the main fluid.
In view of the parasitic recirculation that is observed, it follows that, at a plate that is taken at a given instant, the main fluid that has a given composition arrives, on the one hand, and, on the other hand, the portion of the main fluid that is recirculated from one portion of the sectors to the other sectors of this same plate also arrives, whereby the latter portion has a composition that corresponds to that which the main fluid had one moment beforehand, with the offset in time being equal to dwell time TR of the portion of recirculated fluid.
Everything therefore happens as if a portion of the main fluid reached each plate with a certain delay that is equal to dwell time TR.
The mixing of this portion of recirculated fluid, with a delay, with the main fluid modifies the overall composition of the combined fluid and therefore causes systemic back-mixing at each plate. This induces a deformation, or drag, of the longitudinal concentration sections, and is reflected by a loss of performance such as the reduction in the purity of the extract that can reach, for example, up to a point.
To avoid this problem, it is possible to consider eliminating the circulation of fluids in the chambers by means of nonreturn valves that are arranged on the lines for access to the chambers, but this solution proves to be impractical since the fluid, in the case of a single chamber, can circulate in one direction or the other. Moreover, these valves can pose insoluble maintenance problems due to their inaccessibility.